In a numerical control unit for controlling a machine tool such as a general-purpose milling machine and a lathe to perform machining, an automatic operation mode for controlling the machine tool by executing a machining program and a manual feed mode for feeding a tool by manual operation can be selected. In the manual feed mode, it is possible to perform a manual-continuous feed to continuously drive a machine tool by manual operation through jog feed or the like, a fine adjustment feed for driving the machine tool by operating a manual pulse generator, or an increment feed for moving a tool by a predetermined distance each time a switch is depressed.
In case of performing machining in a desired shape by the manual feed as described above, since a conventional numerical control unit is not equipped with means for preventing excessive cutting, interference or the like in the manual operation, it is necessary for an operator to make machining by paying attention to the excessive cutting, interference or the like. In high-speed machining, since machining is performed at high speed, it is particularly necessary to pay attention to the excessive cutting, interference or the like. Thus, machining takes a long time and requires a skill, so that every operator could not perform machining manually with ease.